Chapter 3 Wireless LANs

Reading materials:[1]Part 4 in textbbok[2]M. Ergen (UC Berkeley), 802.11 tutorial

Outline 3.1 Wireless LAN Technology

3.2 Wireless MAC

3.3 IEEE 802.11 Wireless LAN Standard

3.4 Bluetooth

3.1 Wireless LAN Technology

3.1.1 Overview3.1.2 Infrared LANs3.1.3 Spread Spectrum LANs3.1.4 Narrowband Microwave LANs

3.1.1 Overview WLAN Applications WLAN Requirements WLAN Technology

3.1.1.1 Wireless LAN Applications

LAN Extension Cross-building interconnect Nomadic Access Ad hoc networking

LAN Extension Wireless LAN linked into a wired LAN on

same premises Wired LAN

Backbone Support servers and stationary workstations

Wireless LAN Stations in large open areas Manufacturing plants, stock exchange trading

floors, and warehouses

Multiple-cell Wireless LAN

CM & UM Control module (CM): Interface to a

WLAN, which includes either bridge or router functionality to link the WLAN to the backbone.

User module (UM): control a number of stations of a wired LAN may also be part of the wireless LAN configuration.

Cross-Building Interconnect Connect LANs in nearby buildings

Wired or wireless LANs Point-to-point wireless link is used Devices connected are typically bridges or

routers

Nomadic Access Wireless link between LAN hub and mobile

data terminal equipped with antenna Laptop computer or notepad computer

Uses: Transfer data from portable computer to office

server Extended environment such as campus

Ad Hoc Networking Temporary peer-to-peer network set up to

meet immediate need Example:

Group of employees with laptops convene for a meeting; employees link computers in a temporary network for duration of meeting

3.1.1.2 Wireless LAN Requirements Throughput Number of nodes Connection to backbone LAN Service area Battery power consumption Transmission robustness and security Collocated network operation License-free operation Handoff/roaming Dynamic configuration

3.1.1.3 Wireless LAN Technology

Infrared (IR) LANs Spread spectrum LANs Narrowband microwave

3.1.2 Infrared LANs Strengths and Weakness

Transmission Techniques

Strengths of Infrared Over Microwave Radio Spectrum for infrared virtually unlimited

Possibility of high data rates Infrared spectrum unregulated Equipment inexpensive and simple Reflected by light-colored objects

Ceiling reflection for entire room coverage Doesn’t penetrate walls

More easily secured against eavesdropping Less interference between different rooms

Drawbacks of Infrared Medium Indoor environments experience infrared

background radiation Sunlight and indoor lighting Ambient radiation appears as noise in an

infrared receiver Transmitters of higher power required

Limited by concerns of eye safety and excessive power consumption

Limits range

IR Data Transmission Techniques Directed Beam Infrared Ominidirectional Diffused

Directed Beam Infrared Used to create point-to-point links

(e.g.Fig.13.5) Range depends on emitted power and

degree of focusing Focused IR data link can have range of

kilometers Such ranges are not needed for constructing

indoor WLANs Cross-building interconnect between bridges or

routers

Ominidirectional Single base station within line of sight of all

other stations on LAN Base station typically mounted on ceiling

(Fig.13.6a) Base station acts as a multiport repeater

Ceiling transmitter broadcasts signal received by IR transceivers

Other IR transceivers transmit with directional beam aimed at ceiling base unit

Diffused All IR transmitters focused and aimed at a

point on diffusely reflecting ceiling (Fig.13.6b)

IR radiation strikes ceiling Reradiated omnidirectionally Picked up by all receivers

Typical Configuration for IR WLANs

Fig.13.7 shows a typical configuration for a wireless IR LAN installation

A number of ceiling-mounted base stations, one to a room

Using ceiling wiring, the base stations are all connected to a server

Each base station provides connectivity for a number of stationary and mobile workstations in its area

3.1.3 Spread Spectrum LANs Configuration

Transmission Issues

3.1.3.1 Configuration Multiple-cell arrangement (Figure 13.2) Within a cell, either peer-to-peer or hub Peer-to-peer topology

No hub Access controlled with MAC algorithm

CSMA Appropriate for ad hoc LANs

Spread Spectrum LAN Configuration Hub topology

Mounted on the ceiling and connected to backbone

May control access May act as multiport repeater Automatic handoff of mobile stations Stations in cell either:

Transmit to / receive from hub only Broadcast using omnidirectional antenna

3.1.3.2 Transmission Issues Within ISM band, operating at up

to 1 watt. Unlicensed spread spectrum: 902-

928 MHz (915 MHZ band), 2.4-2.4835 GHz (2.4 GHz band), and 5.725-5.825 GHz (5.8 GHz band). The higher the frequency, the higher the potential bandwidth

3.1.4 Narrowband Microwave LANs

Use of a microwave radio frequency band for signal transmission

Relatively narrow bandwidth Licensed Unlicensed

Licensed Narrowband RF Licensed within specific geographic areas

to avoid potential interference Motorola - 600 licenses (1200 frequencies)

in 18-GHz range Covers all metropolitan areas Can assure that independent LANs in nearby

locations don’t interfere Encrypted transmissions prevent eavesdropping

Unlicensed Narrowband RF RadioLAN introduced narrowband wireless

LAN in 1995 Uses unlicensed ISM spectrum Used at low power (0.5 watts or less) Operates at 10 Mbps in the 5.8-GHz band Range = 50 m to 100 m

3.2 Wireless MAC

Wireless Data Networks Experiencing a tremendous

growth over the last decade or so Increasing mobile work force,

luxury of tetherless computing, information on demand anywhere/anyplace, etc, have contributed to the growth of wireless data

Wireless Network Types … Satellite networks

e.g. Iridium (66 satellites), Qualcomm’s Globalstar (48 satellites)

Wireless WANs/MANs e.g. CDPD, GPRS, Ricochet

Wireless LANs e.g. Georgia Tech’s LAWN

Wireless PANs e.g. Bluetooth

Ad-hoc networks e.g. Emergency relief, military

Sensor networks

Wireless Local Area Networks Probably the most widely used of the

different classes of wireless data networks

Characterized by small coverage areas (~200m), but relatively high bandwidths (upto 50Mbps currently)

Examples include IEEE 802.11 networks, Bluetooth networks, and Infrared networks

WLAN Topology

Distribution Network

MobileStations

Access Point

Static host/Router

Wireless WANs Large coverage areas of upto a

few miles radius Support significantly lower

bandwidths than their LAN counterparts (upto a few hundred kilobits per second)

Examples: CDPD, Mobitex/RAM, Ricochet

WAN Topology

Wireless MAC Channel partitioning techniques

FDMA, TDMA, CDMA Random access

Wireline MAC Revisited ALOHA slotted-ALOHA CSMA CSMA/CD Collision free protocols Hybrid contention-based/collision-

free protocols

Wireless MAC CSMA as wireless MAC? Hidden and exposed terminal

problems make the use of CSMA an inefficient technique

Several protocols proposed in related literature – MACA, MACAW, FAMA

IEEE 802.11 standard for wireless MAC

Hidden Terminal Problem

A talks to B C senses the channel C does not hear A’s transmission (out of range) C talks to B Signals from A and B collide

A B C

Collision

Exposed Terminal Problem

B talks to A C wants to talk to D C senses channel and finds it to be busy C stays quiet (when it could have ideally

transmitted)

A B C D

Notpossible

Hidden and Exposed Terminal Problems Hidden Terminal

More collisions Wastage of resources

Exposed Terminal Underutilization of channel Lower effective throughput

MACA Medium Access with Collision Avoidance Inspired by the CSMA/CA method used by

Apple Localtalk network (for somewhat different reasons)

CSMA/CA (Localtalk) uses a “dialogue” between sender and receiver to allow receiver to prepare for receptions in terms of allocating buffer space or entering “spin loop” on a programmed I/O interface

Basis for MACA In the context of hidden terminal

problem, “absence of carrier does not always mean an idle medium”

In the context of exposed terminal problem, “presence of carrier does not always mean a busy medium”

Data carrier detect (DCD) useless! Get rid of CS (carrier sense) from

CSMA/CA – MA/CA – MACA!!!!

MACA Dialogue between sender and

receiver: Sender sends RTS (request to send) Receiver (if free) sends CTS (clear to

send) Sender sends DATA

Collision avoidance achieved through intelligent consideration of the RTS/CTS exchange

MACA (contd.) When station overhears an RTS

addressed to another station, it inhibits its own transmitter long enough for the addressed station to respond with a CTS

When a station overheads a CTS addressed to another station, it inhibits its own transmitter long enough for the other station to send its data

Hidden Terminal Revisited …

A sends RTS B sends CTS C overheads CTS C inhibits its own transmitter A successfully sends DATA to B

A B CRTS CTS

DATACTS

Hidden Terminal Revisited How does C know how long to wait

before it can attempt a transmission? A includes length of DATA that it wants

to send in the RTS packet B includes this information in the CTS

packet C, when it overhears the CTS packet,

retrieves the length information and uses it to set the inhibition time

Exposed Terminal Revisited

B sends RTS to A (overheard by C) A sends CTS to B C cannot hear A’s CTS C assumes A is either down or out of range C does not inhibit its transmissions to D

A B C DRTS RTS

CTS Cannot hear CTSTx notinhibited

Collisions Still possible – RTS packets can

collide! Binary exponential backoff

performed by stations that experience RTS collisions

RTS collisions not as bad as data collisions in CSMA (since RTS packets are typically much smaller than DATA packets)

Drawbacks Collisions still possible if CTS

packets cannot be heard but carry (transmit) enough to cause significant interference

If DATA packets are of the same size as RTS/CTS packets, significant overheads

MACA Recap No carrier sensing Request-to-send (RTS), Clear-to-

send (CTS) exchange to solve hidden terminal problem

RTS-CTS-DATA exchange for every transmission

MACAW Based on MACA Design based on 4 key observations:

Contention is at receiver, not the sender Congestion is location dependent To allocate media fairly, learning about

congestion levels should be a collective enterprise

Media access protocol should propagate synchronization information about contention periods, so that all devices can contend effectively

Back-off Algorithm MACA uses binary exponential back-off (BEB) BEB: back-off counter doubles after every

collision and reset to minimum value after successful transmission

Unfair channel allocation! Example simulation result:

2 stations A & B communicating with base-station Both have enough packets to occupy entire

channel capacity A gets 48.5 packets/second, B gets 0

packets/second

BEB Unfairness Since successful transmitters reset back-

off counter to minimum value Hence, it is more likely that successful

transmitters continue to be successful Theoretically, if there is no maximum

back-off, one station can get the entire channel bandwidth

Ideally, the back-off counter should reflect the ambient congestion level which is the same for all stations involved!

BEB with Copy MACAW uses BEB with Copy Packet header includes the BEB value used

by transmitter When a station overhears a packet, it copies

the BEB value in the packet to its BEB counter Thus, after each successful transmission, all

stations will have the same backoff counter Example simulation result (same setting as

before: A gets 23.82 packets/second, B gets 23.32

packets/second

MILD adaptation Original back-off scheme uses BEB

upon collision, and resetting back-off to minimum value upon success

Large fluctuations in back-off value Why is this bad? MACAW uses a multiplicative increase

and linear decrease (MILD) scheme for back-off adaptation (with factors of 1.5 and 1 respectively)

Accommodating Multiple Streams

If A has only one queue for all streams (default case), bandwidth will be split as AB:1/4, AC:1/4, DA:1/2

Is this fair? Maintain multiple queues

at A, and contend as if there are two co-located nodes at A

A

B C D

Other modifications (ACK) ACK packet exchange included in

addition to RTS-CTS-DATA Handle wireless (or collision) errors at

the MAC layer instead of waiting for coarse grained transport (TCP) layer retransmission timeouts

For a loss rate of 1%, 100% improvement in throughput demonstrated over MACA

Other modifications (DS) In the exposed terminal scenario (ABCD

with B talking to A), C cannot talk to D (because of the ACK packet introduced)

What if the RTS/CTS exchange was a failure? How does C know this information?

A new packet DS (data send) included in the dialogue: RTS-CTS-DS-DATA-ACK

DS informs other stations that RTS-CTS exchange was successful

Other modifications (RRTS) Request to Request to Send Consider a scenario:

A – B – C – D D is talking to C A sends RTS to B. However, B does not

respond as it is deferring to the D-C transmission

A backs-off (no reply to RTS) and tries later In the meantime if another D-C transmission

begins, A will have to backoff again

RRTS (contd.) The only way A will get access to

channel is if it comes back from a back-off and exactly at that time C-D is inactive (synchronization constraint!)

Note that B can hear the RTS from A! When B detects the end of current D-C

transmission (ACK packet from C to D), it sends an RRTS to A, and A sends RTS

MACAW Recap Backoff scheme

BEB with Copy MILD Multiple streams

New control packets ACK DS RRTS

Other changes (see paper)

IEEE 802.11 The 802.11 standard provides MAC and PHY functionality for

wireless connectivity of fixed, portable and moving stations moving at pedestrian and vehicular speeds within a local area.

Specific features of the 802.11 standard include the following: Support of asynchronous and time-bounded delivery service Continuity of service within extended areas via a Distribution

System, such as Ethernet. Accommodation of transmission rates of 1, 2,10, and 50 Mbps Support of most market applications Multicast (including broadcast) services Network management services Registration and authentication services

IEEE 802.11 The 802.11 standard takes into account

the following significant differences between wireless and wired LANs: Power Management Security Bandwidth Addressing

IEEE 802.11 Topology Independent Basic Service Set

(IBSS) Networks Stand-alone BSS that has no backbone

infrastructure and consists of at-least two wireless stations

Often referred to as an ad-hoc network Applications include single room, sale

floor, hospital wing

IEEE 802.11 Topology (contd.) Extended Service Set (ESS)

Networks Large coverage networks of arbitrary

size and complexity Consists of multiple cells

interconnected by access points and a distribution system, such as Ethernet

IEEE 802.11 Logical Architecture The logical architecture of the 802.11

standard that applies to each station consists of a single MAC and one of multiple PHYs Frequency hopping PHY Direct sequence PHY Infrared light PHY

802.11 MAC uses CSMA/CA (carrier sense multiple access with collision avoidance)

IEEE 802.11 MAC Layer Primary operations

Accessing the wireless medium (!) Joining the network Providing authentication and privacy

Wireless medium access Distributed Coordination Function

(DCF) mode Point Coordination Function (PCF)

mode

IEEE 802.11 MAC (contd.) DCF

CSMA/CA – A contention based protocol PCF

Contention-free access protocol usable on infrastructure network configurations containing a controller called a point coordinator within the access points

Both the DCF and PCF can operate concurrently within the same BSS to provide alternative contention and contention-free periods

CSMA with Collision Avoidance Carrier Sense Multiple Access with

Collision Avoidance (CSMA/CA) Control packet transmissions

precede data packet transmissions to facilitate collision avoidance

4-way (RTS, CTS, Data, ACK) exchange for every data packet transmission

CSMA/CA (Contd.)

A B CRTS

CTS

Data

ACK

C knows B is listeningto A. Will not attempt totransmit to B.

Hidden Terminal Problem Solvedthrough RTS-CTS exchange!

CSMA/CA (Contd.)

Can there be collisions? Control packet collisions (C transmitting RTS at the same time as A) C does not register B’s CTS C moves into B’s range after B’s CTS

CSMA/CA Algorithm Sense channel (CS) If busy

Back-off to try again later Else

Send RTS If CTS not received

Back-off to try again later Else

Send Data If ACK not received

Back-off to try again later Next packet processing

CSMA/CA Algorithm (Contd.) Maintain a value CW (Contention-Window) If Busy,

Wait till channel is idle. Then choose a random number between 0 and CW and start a back-off timer for proportional amount of time (Why?).

If transmissions within back-off amount of time, freeze back-off timer and start it once channel becomes idle again (Why?)

If Collisions (Control or Data) Binary exponential increase (doubling) of CW

(Why?)

Carrier Sensing and Network Allocation Vector Both physical carrier sensing and

virtual carrier sensing used in 802.11 If either function indicates that the

medium is busy, 802.11 treats the channel to be busy

Virtual carrier sensing is provided by the NAV (Network Allocation Vector)

NAV Most 802.11 frames carry a

duration field which is used to reserve the medium for a fixed time period

Tx sets the NAV to the time for which it expects to use the medium

Other stations start counting down from NAV to 0

When NAV > 0, medium is busy

Illustration

Sender

Receiver

NAV

RTS

CTS

DATA

ACK

SIFS

SIFS

SIFS

RTSCTS

Interframe Spacing 802.11 uses 4 different interframe

spacings Interframe spacing plays a large

role in coordinating access to the transmission medium

Varying interframe spacings create different priority levels for different types of traffic!

Types of IFS SIFS

Short interframe space Used for highest priority transmissions

– RTS/CTS frames and ACKs DIFS

DCF interframe space Minimum idle time for contention-

based services (> SIFS)

Types (contd.) PIFS

PCF interframe space Minimum idle time for contention-free

service (>SIFS, <DIFS) EIFS

Extended interframe space Used when there is an error in

transmission

Power Saving Mode (PS) 802.11 stations can maximize battery life by

shutting down the radio transceiver and sleeping periodically

During sleeping periods, access points buffer any data for sleeping stations

The data is announced by subsequent beacon frames

To retrieve buffered frames, newly awakened stations use PS-poll frames

Access point can choose to respond immediately with data or promise to delivery it later

IEEE 802.11 MAC Frame Format Overall structure:

Frame control (2 octets) Duration/ID (2 octets) Address 1 (6 octets) Address 2 (6 octets) Address 3 (6 octets) Sequence control (2 octets) Address 4 (6 octets) Frame body (0-2312 octets) FCS (4 octets)

Other MAC Schemes FAMA

Floor Acquisition Multiple Access Prevents any data collisions

MACA-BI MACA by invitation No RTS but CTS retained Suitable for multi-hop wireless

networks Several other approaches …

Other MAC standards HiperLAN (1/2)

Radio channel accessed on a centralized time-sharing basis

TDMA/TDD with all communication coordinated by a central entity

HiSWANa Combines key features of 802.11 and

HiperLAN at the expense of increased overheads

Satellite MAC PRMA: Packet Reservation Multiple Access Combination of TDMA and slotted-ALOHA Satellite channel consists of multiple time

slots in a framed structure Assignment of time slots not done

statically, but in real-time dynamically Each packet identifies the receiving

station uniquely

Satellite MAC (contd.) Slots classified as reserved and free Mobile terminal that needs new slot

contends in one of the free slots If it succeeds, it gains access to that

particular slot thereafter A mobile terminal implicitly relinquishes a

slot when it does not transmit anything in that slot

If collision occurs during contention for a free slot, traditional back-off algorithms used (e.g. binary exponential back-off)

PRMA (contd.) Suitable for LEO satellites where round-trip

time is reasonable (for mobile terminal to know if it has gotten access to a particular slot)

FRMA: Frame reservation multiple access – satellite base-station replies only at the end of a frame (as opposed to the end of a slot) to convey successful capture of a slot

Hybrid PRMA/TDMA possible for traffic with QoS requirements

Most modern satellite systems use CDMA

Recap Random Access MAC Schemes

CSMA MACA MACAW IEEE 802.11 Standard

3.3 IEEE 802.11 Wireless LAN Standard

Outline IEEE 802 Architecture 802.11 Architecture and Services 802.11 MAC 802.11 Physical Layer Other 802.11 Standards

3.3 .1 IEEE 802 Architecture

IEEE 802 Protocol Layers

Protocol Architecture Functions of physical layer:

Encoding/decoding of signals Preamble generation/removal (for

synchronization) Bit transmission/reception Includes specification of the transmission

medium

Protocol Architecture Functions of medium access control (MAC) layer:

On transmission, assemble data into a frame with address and error detection fields

On reception, disassemble frame and perform address recognition and error detection

Govern access to the LAN transmission medium Functions of logical link control (LLC) Layer:

Provide an interface to higher layers and perform flow and error control

Separation of LLC and MAC The logic required to manage access to a

shared-access medium not found in traditional layer 2 data link control

For the same LLC, several MAC options may be provided

MAC Frame Format MAC control

Contains Mac protocol information Destination MAC address

Destination physical attachment point Source MAC address

Source physical attachment point CRC

Cyclic redundancy check

Logical Link Control Characteristics of LLC not shared by other

control protocols: Must support multiaccess, shared-medium

nature of the link Relieved of some details of link access by

MAC layer

LLC Services Unacknowledged connectionless service

No flow- and error-control mechanisms Data delivery not guaranteed

Connection-mode service Logical connection set up between two users Flow- and error-control provided

Acknowledged connectionless service Cross between previous two Datagrams acknowledged No prior logical setup

Differences between LLC and HDLC LLC uses asynchronous balanced mode of

operation of HDLC (type 2 operation) LLC supports unacknowledged

connectionless service (type 1 operation) LLC supports acknowledged connectionless

service (type 3 operation) LLC permits multiplexing by the use of

LLC service access points (LSAPs)

3.3.2 IEEE 802.11 Architecture and Services

3.3.2.1 The Wi-Fi Alliance Wi-Fi: Wireless Fidelity WECA: Wireless Ethernet

Compatibility Alliance, an industry consortium formed in 1999

3.3.2.2 IEEE 802.11 Architecture Distribution system (DS) Access point (AP) Basic service set (BSS)

Stations competing for access to shared wireless medium

Isolated or connected to backbone DS through AP Extended service set (ESS)

Two or more basic service sets interconnected by DS

3.3.2.3 IEEE 802.11 Services

Distribution of Messages Within a DS Distribution service

Used to exchange MAC frames from station in one BSS to station in another BSS

Integration service Transfer of data between station on IEEE

802.11 LAN and station on integrated IEEE 802.x LAN

Transition Types Based On Mobility No transition

Stationary or moves only within BSS BSS transition

Station moving from one BSS to another BSS in same ESS

ESS transition Station moving from BSS in one ESS to BSS

within another ESS

Association-Related Services Association

Establishes initial association between station and AP Reassociation

Enables transfer of association from one AP to another, allowing station to move from one BSS to another

Disassociation Association termination notice from station or AP

Access and Privacy Services Authentication

Establishes identity of stations to each other Deathentication

Invoked when existing authentication is terminated

Privacy Prevents message contents from being read by

unintended recipient

3.3.3 IEEE 802.11 MAC

IEEE 802.11 Medium Access Control MAC layer covers three functional areas:

Reliable data delivery Access control Security

3.3.3.1 Reliable Data Delivery More efficient to deal with errors at the MAC level than

higher layer (such as TCP) Frame exchange protocol

Source station transmits data Destination responds with acknowledgment (ACK) If source doesn’t receive ACK, it retransmits frame

Four frame exchange Source issues request to send (RTS) Destination responds with clear to send (CTS) Source transmits data Destination responds with ACK

3.3.3.2 Medium Access Control

DCF (Distributed Coordination Function)PCF (Point Coordination Function)MAC Frame

Access Control

Distributed Coordination Function

DCF makes use of a simple CSMA (carrier sense multiple access) algorithm

Medium Access Control Logic

Interframe Space (IFS) Values Short IFS (SIFS)

Shortest IFS Used for immediate response actions

Point coordination function IFS (PIFS) Midlength IFS Used by centralized controller in PCF scheme when using

polls Distributed coordination function IFS (DIFS)

Longest IFS Used as minimum delay of asynchronous frames contending

for access

IFS Usage SIFS

Acknowledgment (ACK) Clear to send (CTS) Poll response

PIFS Used by centralized controller in issuing polls Takes precedence over normal contention traffic

DIFS Used for all ordinary asynchronous traffic

Point Coordination Function

PCF is on top of DCFThe operation consists of polling by the point coordinatorThe point coordinator makes use of PIFS when issuing polls. PIFS is smaller than DIFS, the point coordinator can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses

MAC Frame

MAC Frame Format

MAC Frame Fields Frame Control – frame type, control information Duration/connection ID – channel allocation time Addresses – context dependant, types include

source and destination Sequence control – numbering and reassembly Frame body – MSDU or fragment of MSDU Frame check sequence – 32-bit CRC

Frame Control Fields Protocol version – 802.11 version Type – control, management, or data Subtype – identifies function of frame To DS – 1 if destined for DS From DS – 1 if leaving DS More fragments – 1 if fragments follow Retry – 1 if retransmission of previous frame

Frame Control Fields Power management – 1 if transmitting station is in

sleep mode More data – Indicates that station has more data to

send WEP – 1 if wired equivalent protocol is

implemented Order – 1 if any data frame is sent using the

Strictly Ordered service

Control Frame Subtypes Power save – poll (PS-Poll) Request to send (RTS) Clear to send (CTS) Acknowledgment Contention-free (CF)-end CF-end + CF-ack

Data Frame Subtypes Data-carrying frames

Data Data + CF-Ack Data + CF-Poll Data + CF-Ack + CF-Poll

Other subtypes (don’t carry user data) Null Function CF-Ack CF-Poll CF-Ack + CF-Poll

Management Frame Subtypes Association request Association response Reassociation request Reassociation response Probe request Probe response Beacon

Management Frame Subtypes Announcement traffic indication message Dissociation Authentication Deauthentication

3.3.4 802.11 Physical Layer

Overview The physical layer for IEEE 802.11

has been issued in four stages. 802.11, 802.11a, 802.11b, 802.11g

Original 802.11 Physical Layer DSSS FHSS Infrared

Physical Media Defined by Original 802.11 Standard Direct-sequence spread spectrum

Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps

Frequency-hopping spread spectrum Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps

Infrared 1 and 2 Mbps Wavelength between 850 and 950 nm

IEEE 802.11a Channel Structure Coding and Modulation Physical-Layer Frame Structure

Channel Structure 802.11a makes use of the

frequency band called the UNNI (Universal Networking Information Infrastructure)

UNNI includes UNNI-1(5.15-5.25GHz, indoor use), UNNI-2(5.25-5.35GHz, indoor or outdoor use), and UNNI-3(5.725-5.825GHz, outdoor use)

Coding and Modulation OFDM: Orthogonal Frequency

Division Multiplexing, uses multiple carrier signals at different frequencies, sending some of bits on each channel. Similar to FDM, However, in the case of OFDM, all of the subchannels are dedicated to a single data source.

Physical-Layer Frame Structure

IEEE 802.11b CCK Modulation Scheme Physical-Layer Frame Structure

(Fig. 14.11 (b))

CCK 802.11b is an extension of the

802.11 DSSS scheme, providing data rates of 5.5 and 11 Mbps in the ISM band.

Modulation scheme is CCK (Complementary code keying)

802.11g

Speed vs Distance

3.3.5 Other IEEE 802.11 Standards

802.11c802.11d802.11e802.11f802.11h802.11i802.11k802.11m802.11n

802.11c is concerned with bridge operation

802.11d deals with issues related to regulatory differences in various countries

802.11e makes revisions to the MAC layer to improve quality of service and address some security issues

802.11f addresses the issue of interoperability among access points (APs) from multiple vendors

802.11h deals with spectrum and power management issues

802.11i defines security and authentication mechanisms at the MAC layer

802.11k defines Radio Resource Management enhancements to provide mechanisms to higher layers for radio and network measurements

802.11m is an ongoing task group activity to correct editorial and technical issues in the standard

802.11n is studying a range of enhancements to both the physical and MAC layers to improve throughput

3.4 Bluetooth Techniques

Reading material:[1]Investigation into Bluetooth Technology, Jean Parrend, Liverpool John Moores University

3.4.1 Overview Universal short-range wireless capability Uses 2.4-GHz band Available globally for unlicensed users Devices within 10 m can share up to 720

kbps of capacity Supports open-ended list of applications

Data, audio, graphics, video

Bluetooth Application Areas Data and voice access points

Real-time voice and data transmissions Cable replacement

Eliminates need for numerous cable attachments for connection

Ad hoc networking Device with Bluetooth radio can establish

connection with another when in range

Bluetooth Standards Documents Core specifications

Details of various layers of Bluetooth protocol architecture

Profile specifications Use of Bluetooth technology to support various

applications

Protocol Architecture Bluetooth is a layered protocol architecture

Core protocols Cable replacement and telephony control protocols Adopted protocols

Core protocols Radio Baseband Link manager protocol (LMP) Logical link control and adaptation protocol (L2CAP) Service discovery protocol (SDP)

Protocol Architecture Cable replacement protocol

RFCOMM Telephony control protocol

Telephony control specification – binary (TCS BIN) Adopted protocols

PPP TCP/UDP/IP OBEX WAE/WAP

Usage Models File transfer Internet bridge LAN access Synchronization Three-in-one phone Headset

Piconets and Scatternets Piconet

Basic unit of Bluetooth networking Master and one to seven slave devices Master determines channel and phase

Scatternet Device in one piconet may exist as master or slave in

another piconet Allows many devices to share same area Makes efficient use of bandwidth

3.4.2 Radio Specification

Classes of transmitters Class 1: Outputs 100 mW for maximum

range Power control mandatory Provides greatest distance

Class 2: Outputs 2.4 mW at maximum Power control optional

Class 3: Nominal output is 1 mW Lowest power

3.4.3 Baseband Specification

Frequency Hopping in Bluetooth Provides resistance to interference and

multipath effects Provides a form of multiple access among

co-located devices in different piconets

Frequency Hopping Total bandwidth divided into 1MHz physical channels FH occurs by jumping from one channel to another in

pseudorandom sequence; The FH sequence is determined by the master in a piconet and is a function of the master’s Bluetooth address

Hopping sequence shared with all devices on piconet Piconet access:

Bluetooth devices use time division duplex (TDD) Access technique is TDMA FH-TDD-TDMA

Frequency Hopping

Physical Links between Master and Slave Synchronous connection oriented (SCO)

Allocates fixed bandwidth between point-to-point connection of master and slave

Master maintains link using reserved slots Master can support three simultaneous links

Asynchronous connectionless (ACL) Point-to-multipoint link between master and all slaves Only single ACL link can exist

Bluetooth Packet Fields Access code – used for timing

synchronization, offset compensation, paging, and inquiry

Header – used to identify packet type and carry protocol control information

Payload – contains user voice or data and payload header, if present

Types of Access Codes Channel access code (CAC) – identifies a

piconet Device access code (DAC) – used for

paging and subsequent responses Inquiry access code (IAC) – used for

inquiry purposes

Access Code Preamble – used for DC compensation

0101 if LSB of sync word is 0 1010 if LSB of synch word is 1

Sync word – 64-bits, derived from: 7-bit Barker sequence; including a bit in LAP Lower address part (LAP); 24bits; each Bluetooth device is

assigned a globally unique 48-bit address Pseudonoise (PN) sequence; 64 bits but using 30 bits Taking the bitwise (LAP + Baker code), PN, and data to obtain

the scrambled information; adding 34 check bits with BCH and taking the bitwise XOR with PN

Trailer 0101 if MSB of sync word is 1 1010 if MSB of sync word is 0

Packet Header Fields AM_ADDR – contains “active mode” address of one of

the slaves; temporary address assigned to a slave in this piconet

Type – identifies type of packet (Table 15.5); HVx packets carry 64-kbps voice with different amounts of error protection; DV packets carry both voice and data, DMx or DHx packets carry data (Table 15.4)

Flow – 1-bit flow control; for ACL traffic only ARQN – 1-bit acknowledgment; for ACL traffic

protected by a CRC (Table 15.5) SEQN – 1-bit sequential numbering schemes Header error control (HEC) – 8-bit error detection code

Payload Format Payload header

L_CH field – identifies logical channel Flow field – used to control flow at L2CAP

level Length field – number of bytes of data

Payload body – contains user data CRC – 16-bit CRC code

Error Correction Schemes 1/3 rate FEC (forward error correction)

Used on 18-bit packet header, voice field in HV1 packet

2/3 rate FEC Used in DM packets, data fields of DV packet,

FHS packet and HV2 packet ARQ

Used with DM and DH packets

ARQ Scheme Elements Error detection – destination detects errors,

discards packets Positive acknowledgment – destination returns

positive acknowledgment Retransmission after timeout – source retransmits

if packet unacknowledged Negative acknowledgment and retransmission –

destination returns negative acknowledgement for packets with errors, source retransmits

Fast ARQ Bluetooth uses the fast ARQ scheme,

which takes advantage of the fact that a master and slave communicate in alternate time slots

Fig. 15.9 illustrates the technique Fig. 15.10 shows the ARQ mechanism

in more detail

Logical Channels Link control (LC) Link manager (LM) User asynchronous (UA) User isochronous (UI) User synchronous (US)

Logical Channels—LC Used to manage the flow of packets over the link

interface. The LC channel is mapped onto the packet header. This channel carries low-level link control information like ARQ, flow control, and payload characterization. The LC channel is carried in every packet except in the ID packet, which has no packet header

Logical Channels—LM Transports link management information

between participating stations. This logical channel supports LMP traffic and can be carried over either an SCO or ACL link

Logical Channels—UA Carries asynchronous user data. This

channel is normally carried over the ACL link but may be carried in a DV packet on the SCO link

Logical Channels—UI Carries isochronous user data, which recurs

with known periodic timing. This channel is normally carried over the ACL link but may be carried in a DV packet on the SCO link. At the baseband level, the UI channel is treated the same way as a UA channel. Timing to provide isochronous properties is provided at a higher layer

Logical Channels—US Carries synchronous user data. This channel

is carried over the SCO link

Channel Control States of operation of a piconet during link

establishment and maintenance Major states

Standby – default state Connection – device connected

Channel Control Interim substates for adding new slaves

Page – device issued a page (used by master) Page scan – device is listening for a page Master response – master receives a page response

from slave Slave response – slave responds to a page from master Inquiry – device has issued an inquiry for identity of

devices within range Inquiry scan – device is listening for an inquiry Inquiry response – device receives an inquiry response

Inquiry Procedure Potential master identifies devices in range that

wish to participate Transmits ID packet with inquiry access code (IAC) Occurs in Inquiry state

Device receives inquiry Enter Inquiry Response state Returns FHS packet with address and timing

information Moves to page scan state

Page Procedure Master uses devices address to calculate a

page frequency-hopping sequence Master pages with ID packet and device

access code (DAC) of specific slave Slave responds with DAC ID packet Master responds with its FHS packet Slave confirms receipt with DAC ID Slaves moves to Connection state

Slave Connection State Modes Active – participates in piconet

Listens, transmits and receives packets Sniff – only listens on specified slots Hold – does not support ACL packets

Reduced power status May still participate in SCO exchanges

Park – does not participate on piconet Still retained as part of piconet

Bluetooth Audio Voice encoding schemes:

Pulse code modulation (PCM) Continuously variable slope delta (CVSD)

modulation Choice of scheme made by link manager

Negotiates most appropriate scheme for application

3.4.4 Link Manager Specification

LMP PDUs General response Security Service

Authentication Pairing Change link key Change current link key Encryption

LMP PDUs Time/synchronization

Clock offset request Slot offset information Timing accuracy information request

Station capability LMP version Supported features

LMP PDUs Mode control

Switch master/slave role Name request Detach Hold mode Sniff mode Park mode Power control

LMP PDUs Mode control (cont.)

Channel quality-driven change between DM and DH

Quality of service Control of multislot packets Paging scheme Link supervision

3.4.5 Logical Link Control and Adaptation Protocol

L2CAP Provides a link-layer protocol between entities

with a number of services Relies on lower layer for flow and error control Makes use of ACL links, does not support SCO

links Provides two alternative services to upper-layer

protocols Connection service Connection-mode service

L2CAP Logical Channels Connectionless

Supports connectionless service Each channel is unidirectional Used from master to multiple slaves

Connection-oriented Supports connection-oriented service Each channel is bidirectional

Signaling Provides for exchange of signaling messages between

L2CAP entities

L2CAP Packet Fields for Connectionless Service Length – length of information payload, PSM

fields Channel ID – 2, indicating connectionless channel Protocol/service multiplexer (PSM) – identifies

higher-layer recipient for payload Not included in connection-oriented packets

Information payload – higher-layer user data

Signaling Packet Payload Consists of one or more L2CAP commands,

each with four fields Code – identifies type of command Identifier – used to match request with reply Length – length of data field for this command Data – additional data for command, if

necessary

L2CAP Signaling Command Codes

L2CAP Signaling Commands Command reject command

Sent to reject any command Connection commands

Used to establish new connections Configure commands

Used to establish a logical link transmission contract between two L2CAP entities

L2CAP Signaling Commands Disconnection commands

Used to terminate logical channel Echo commands

Used to solicit response from remote L2CAP entity

Information commands Used to solicit implementation-specific

information from remote L2CAP entity

Flow Specification Parameters Service type Token rate (bytes/second) Token bucket size (bytes) Peak bandwidth (bytes/second) Latency (microseconds) Delay variation (microseconds)

3.4.6 IEEE 802.15

WPAN 802.15 is for short range WPANs

(Wireless Personal Area Networks) A PAN is communication network within

a small area in which all of the devices on the network are typically owned by one person or perhaps a family

IEEE 802.15.3 Concerned with the high data rate

WPANs

Examples of Applications Connecting digital still cameras to printers or

kiosks Laptop to projector connection Connecting a personal digital assistant (PDA)

to a camera or PDA to a printer Speakers in a 5:1 surround-sound system

connecting to the receiver Video distribution from a set-top box or cable

modem Sending music from a CD or MP3 player to

headphones or speakers Video camera display on television Remote view finders for video or digital still

cameras

Requirements of Applications

Short range: 10m High throughput: greater than 20 Mbps Low power usage Low cost QoS capable Dynamic environment: for mobile device,

a speed of less than 7 km/h is addressed Simple connectivity privacy

MAC of 802.15.3 An 802.15.3 network consists of a collection

of devices (DEVs). One of the DEVs also acts as a piconet

coordinator (PNC) The PNC assigns time for connections

between DEVs All commands are between the PNC and DEVs The PNC is used to control access to the time

resources of the piconet and is not involved in the exchange of data frames between DEVs

Physical Layer of 802.15.3

IEEE 802.15.3a Provides a higher speed (110Mbps

or greater) PHY amendment to the draft P802.15.3 standard

The new PHY will use the P802.15.3 MAC with limited modification

IEEE 802.15.4 Investigates a low data solution

with mutimonth to multiyear battery life and very low complexity

PHYs: 868 MHz/915 MHz DSSS, 2.4 GHz DSSS